Feline hyperthyroidism 2009 (Proceedings)

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Feline hyperthyroidism 2009 (Proceedings)

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Apr 01, 2009

Today's feline practitioners are well-versed in the pathophysiology, diagnosis, and treatment of hyperthyroidism. This disease as one of the most commonly diagnosed disorders of geriatric cats, and as our knowledge and experience in diagnosing and treating the disease have grown, certain challenges have become apparent. Treatment options for feline hyperthyroidism have not changed substantially in recent years, but awareness of adverse effects of treatment has grown, especially regarding the effects of treatment on renal function.

The Effects of Treatment on Renal Function

Several studies have shown that treatment of hyperthyroidism can have profound effects of kidney function in cats, and that feline hyperthyroidism can mask underlying chronic renal failure (Graves et al.1994, DiBartola et al. 1996; Adams et al. 1997, Becker et al. 2000). Treatment of hyperthyroidism by bilateral surgical thyroidectomy causes a consistent, and sometimes disastrous, drop in glomerular filtration rate (GFR). The same is true for radioiodine treatment. For these reasons, the wisdom of treating hyperthyroidism in cats with questionable renal function has been questioned. Especially in cats with impaired renal function, the used of methimazole seems an attractive first-line treatment for hyperthyroidism. Treatment with methimazole would be reversible in the event of exacerbation of renal failure. Furthermore, methimazole has actually been shown to have beneficial effects on kidney function in some species. However, since the effects of methimazole on renal function in hyperthyroid cats has not been studied, treatment recommendations are based on speculation. In fact, we found that methimazole consistently causes a decrease in GFR in hyperthyroid cats, and that the drop can often unmask underlying chronic renal failure (Becker et al. 2000). Fortunately, we were also able to demonstrate a reversal of this effect upon cessation of methimazole therapy. Other investigators have studied the effects of radioiodine treatment on renal function, and have found that it too causes a decline in renal function in hyperthyroid cats.

Because so many cats exhibit declining renal function upon treatment of hyperthyroidism, the currently recommended first-line treatment is methimazole, for the simple reason that it is reversible. If a cat treated with methimazole experiences a severe drop in GFR and develops overt renal failure, the clinician must begin the difficult course of balancing hyperthyroidism and its effects against the effects of renal failure. In some cases, hyperthyroidism cannot be treated at all, and the clinician must just manage weight loss, hypertension, tachycardia, and cardiac disease. In cases in which methimazole does not cause overt renal failure, owners and veterinarians can probably feel more comfortable about pursuing a more permanent treatment such as radioiodine or surgical thyroidectomy.

Methimazole Trials

In recent years, investigators have shown consistently that treatment of hyperthyroidism can have profound effects of kidney function in cats, and that feline hyperthyroidism can mask underlying chronic renal insufficiency. Post-treatment renal insufficiency is common, occurring in 17% to 38% of cats treated for hyperthyroidism. Treatment of hyperthyroidism by bilateral surgical thyroidectomy causes a consistent, and sometimes disastrous, drop in glomerular filtration rate (GFR) (Graves et al. 1994)). The same is true for radioiodine treatment and methimazole (DiBartola et al. 1996; Adams et al. 1997, Becker et al. 2000). For these reasons, the wisdom of treating hyperthyroidism in cats with questionable renal function has been questioned. Especially in cats with impaired renal function, the use of methimazole is the most logical first-line treatment for hyperthyroidism. Treatment with methimazole can be reversed in the event of exacerbation of renal insufficiency. For this reason, a trial with methimazole is recommended prior to a more permanent treatment for hyperthyroidism (Graves 1997).